Imaging Optical System

The present invention relates to an imaging optical system suitable for an imaging lens used in an imaging apparatus such as a digital camera or a video camera.

In an imaging lens used in an imaging apparatus such as a digital camera, an imaging optical system having a large aperture ratio with a bright F-number is required for reasons such as an increase in the amount of blurriness before and after a focus distance of an object, which widens the width of image expression that takes advantage of blurriness, and an increase in a degree of suppression of camera shake and subject shake due to a decrease in an exposure time. In the related art, for example, the following patent documents are known as imaging optical systems having a large aperture ratio.

In recent years, in an imaging lens used in an imaging apparatus such as a digital camera, it is required to have high image formation performance with an increase in the number of pixels of an image sensor. In addition, with an increase in the number of small and lightweight cameras due to mirrorless cameras, there is a demand for a reduction in size and weight of an imaging optical system of an imaging lens combined with the camera. It is also required to achieve high performance as well as small size and light weight, in an imaging optical system with a large aperture ratio.

In addition, in recent years, there is a demand for high-speed auto focus and quietness during focus driving, and it is advantageous to reduce the weight of the focus lens group for this purpose. In addition, in recent years, capturing of a video using a digital camera has become common. In an imaging optical system used for video capturing, it is required to suppress a change in angle of view in a case where the focusing is changed to different focusing positions, so-called focus breathing.

The optical system disclosed in Patent Document 1 realizes high image formation performance with a large aperture ratio with a maximum aperture of about F1.25, and in some examples, the amount of focus breathing is also small. However, there is a problem in that it is difficult to achieve sufficient reduction in size and weight because the total length of the entire optical system is large and the weight of the glass material is heavy. In addition, there is a problem in that the number of lenses in the focus lens group is large and it is difficult to reduce the weight of the focus lens group. The optical system disclosed in Patent Document 2 realizes high image formation performance with a maximum aperture of about F1.44, which is a large aperture ratio, and further suppresses the total length of the entire optical system and the weight of the glass material. However, there is a problem in that focus breathing during focusing is large, and the number of lenses in the focus lens group is large, making it difficult to reduce the weight of the focus lens groups.

The present invention has been made in view of such circumstances, and an object of the present invention is to provide an imaging optical system that achieves both high imaging performance and reduction in size and weight while having a large aperture ratio, has a lightweight focus lens group, and suppresses focus breathing during focusing.

In order to achieve the above object, an imaging optical system according to an aspect of the present invention includes, in order from an object side to an image side, a first lens group G, a second lens group G, a third lens group Ghaving a positive refractive power, a fourth lens group Ghaving a positive refractive power, and a fifth lens group Ghaving a negative refractive power, in which, when focusing from an infinite distance object to a close distance object, the first lens group Gremains stationary with respect to an image surface, the second lens group Gmoves to the object side along an optical axis, the third lens group Gremains stationary with respect to the image surface, the fourth lens group Gmoves to the object side along the optical axis, and the fifth lens group Gremains stationary with respect to the image surface, the imaging optical system includes an aperture diaphragm S between a lens surface in the second lens group Gclosest to the image side and a lens surface in the fourth lens group Gclosest to the object side, and the imaging optical system satisfies a conditional expression shown below.

Further, an imaging optical system according to an aspect of the present invention includes, in order from an object side to an image side, a first lens group G, a second lens group G, a third lens group Ghaving a positive refractive power, a fourth lens group Ghaving a positive refractive power, and a fifth lens group Ghaving a negative refractive power, in which, when focusing from an infinite distance object to a close distance object, the first lens group Gremains stationary with respect to an image surface, the second lens group Gmoves to the object side along an optical axis, the third lens group Gremains stationary with respect to the image surface, the fourth lens group Gmoves to the object side along the optical axis, and the fifth lens group Gremains stationary with respect to the image surface, the imaging optical system includes an aperture diaphragm S between a lens surface in the second lens group Gclosest to the image side and a lens surface in the fourth lens group Gclosest to the object side, the fifth lens group Gincludes at least one positive lens, and the imaging optical system satisfies conditional expressions shown below.

According to the imaging optical system of the present invention, it is possible to provide an imaging optical system that achieves both high imaging performance and reduction in size and weight while having a large aperture ratio, has a lightweight focus lens group, and suppresses focus breathing during focusing.

Hereinafter, embodiments of the present invention will be described. In a case where refractive indices for a g line (wavelength: 435.8 nm), an F line (wavelength: 486.1 nm), a d line (wavelength: 587.6 nm), and a C line (wavelength: 656.3 nm) are respectively denoted by ng, nF, nd, and nC, an Abbe number νd and a partial dispersion ratio θgF are represented by the following expressions. νd=(nd−1)/(nF−nC)

As can be seen from the lens configuration diagrams shown in, the imaging optical system of the present invention includes a first lens group G, a second lens group G, a third lens group Ghaving a positive refractive power, a fourth lens group Ghaving a positive refractive power, and a fifth lens group Ghaving a negative refractive power in order from the object side, and has a configuration in which, in a case where focusing on the close distance object from the infinite distance object is performed, the first lens group Gremains stationary with respect to the image surface, the second lens group Gmoves to the object side along the optical axis, the third lens group Gremains stationary with respect to the image surface, the fourth lens group Gmoves to the object side along the optical axis, the fifth lens group Gremains stationary with respect to the image surface, and an aperture diaphragm S is provided between the lens surface in the second lens group Gclosest to the image side and the lens surface in the fourth lens group Gclosest to the object side.

In a lens configuration in which the fourth lens group Ghaving a positive refractive power, which is disposed closer to the image surface side than the aperture diaphragm S during focusing from the infinite distance object to the close distance object, is extended to the object side along the optical axis, it is easy to suppress the focus breathing by disposing the fourth lens group Gat a position away from the aperture diaphragm S. On the other hand, in a case where the ray height of the peripheral principal ray is increased, the fluctuation of aberration during focusing, particularly, the fluctuation of astigmatism and comatic aberration increases. In a case where the number of lenses is increased in order to suppress the fluctuation in aberration, the size of the entire optical system is increased.

Therefore, by extending the second lens group Gdisposed closer to the object side than the aperture diaphragm S to the object side along the optical axis in addition to the fourth lens group Gduring focusing from the infinite distance object to the close distance object to offset the fluctuation in aberration during focusing, it is easy to achieve both the suppression of fluctuation in aberration during focusing and the suppression of focus breathing while reducing the size of the entire optical system.

In addition, since the first lens group Gpositioned closest to the object side and the fifth lens group Gpositioned closest to the image side in the optical system are stationary with respect to the image surface during focusing, and a structure in which lenses that move during focusing are not exposed to the outside is adopted, it is easy to improve dustproof and waterproof performance. Since the fifth lens group Ghas a negative refractive power, the position of the exit pupil can be brought closer to the image surface side, and it is easy to suppress the amount of vignetting of peripheral rays due to the constraint of the mount diameter of the camera.

Furthermore, the imaging optical system of the present invention satisfies the conditional expression below.

Conditional Expression (1) specifies a preferable range for appropriate disposition of the second lens group Gand the fourth lens group G. As described above, in order to suppress the focus breathing, it is desirable that the fourth lens group Gis disposed at a position appropriately away from the aperture diaphragm S toward the image side, and it is desirable that the second lens group Gis disposed at a position appropriately away from the aperture diaphragm S toward the object side so that the second lens group Ghas a certain degree of height of the peripheral principal ray in order to offset the astigmatism and comatic aberration occurring due to the movement of the fourth lens group Gduring focusing. In order for both the second lens group Gand the fourth lens group Gto appropriately keep their distances from the aperture diaphragm S, it is necessary to appropriately set the distance between the second lens group Gand the fourth lens group G.

In a case where the distance between the second lens group Gand the fourth lens group Gis decreased below the lower limit value of Conditional Expression (1), it is difficult to achieve both the suppression of the focus breathing and the suppression of the fluctuation in aberration during focusing. On the other hand, in a case where the distance between the second lens group Gand the fourth lens group Gincreases beyond the upper limit value of Conditional Expression (1), it is difficult to dispose the first lens group Gand the fifth lens group Gfixed during focusing, or the fourth lens group Gis disposed at a position where the radial constraint near the mount is severe, and it is difficult to dispose the focus actuator.

Regarding Conditional Expression (1), the above-described effect can be made more reliable by desirably limiting the lower limit value to 0.35 and the upper limit value to 0.60.

Furthermore, in the imaging optical system of the present invention, the fifth lens group Gincludes at least one positive lens. By having the positive lens, it is easy to suppress chromatic aberration occurring in the fifth lens group Ghaving a negative refractive power.

In addition, it is desirable to satisfy the conditional expressions shown below.

Conditional Expressions (2) and (3) specify preferable characteristics for satisfactorily correcting chromatic aberration including a second spectrum for the material of at least one positive lens included in the fifth lens group G. Since the fifth lens group Gis disposed at a position close to the image surface, the ray height of the peripheral principal ray is high, and the on-axis luminous flux diameter is also large in the imaging optical system having a large aperture ratio. Therefore, the lenses in the fifth lens group Gcontribute to the correction of both the lateral chromatic aberration and the on-axis chromatic aberration.

In order to satisfactorily correct chromatic aberration including a secondary spectrum, it is desirable to use a material having high anomalous dispersion (having a large partial dispersion ratio compared to a normal material) for a positive lens. The optical glass that actually exists as a material having particularly high anomalous dispersion is roughly classified into glass having a low refractive index and low dispersion and glass having a high refractive index and high dispersion. The fifth lens group Ghas a negative refractive power. In a case where a material having a low refractive index and low dispersion is used for the positive lens, the primary color correction in the fifth lens group Gis insufficient. Therefore, it is preferable to select a material having a high refractive index and high dispersion. In addition, by using a material having a high refractive index for the positive lens, it is also easy to suppress the Petzval sum of the entire lens system and to satisfactorily correct the astigmatism.

In a case where the anomalous dispersion of at least one positive lens included in the fifth lens group Gis decreased below the lower limit value of Conditional Expression (2), it is difficult to satisfactorily correct the longitudinal chromatic aberration and the lateral chromatic aberration including the secondary spectrum.

In Conditional Expression (2), desirably, by limiting the lower limit value to 0.0300, the above-described effect can be made more reliable.

In a case where the Abbe number of at least one positive lens included in the fifth lens increases beyond the upper limit value of Conditional Expression (3), the optical glass satisfying Conditional Expression (2) is limited to the optical glass in a low refractive index and low dispersion region at the present time, the first-order color correction in the fifth lens group Gis insufficient, and it is difficult to suppress the Petzval sum of the entire lens system and satisfactorily correct the astigmatism.

In Conditional Expression (3), by desirably limiting the upper limit value to 20.50, the above-described effect can be made more reliable.

Furthermore, in the imaging optical system of the present invention, the fifth lens group Gincludes at least two positive lenses. In a case where only one positive lens that satisfies Conditional Expressions (2) and (3) is provided in the fifth lens group G, the balance of chromatic aberration is largely changed by slightly changing the power of the positive lens because the Abbe number is small. Therefore, the degree of freedom of correction of various aberrations such as spherical aberration other than chromatic aberration is low. Therefore, by disposing a positive lens in addition to the positive lens satisfying Conditional Expressions (2) and (3) in the fifth lens group G, it is easy to achieve both correction of chromatic aberration and correction of other aberrations.

In addition, the following conditional expressions are satisfied.

Conditional Expression (4) is for specifying a preferable range of a difference in an Abbe number between a positive lens having a maximum Abbe number and a positive lens having a minimum Abbe number among at least two positive lenses of the fifth lens group G.

In a case where the difference in the Abbe number between the positive lens having the maximum Abbe number and the positive lens having the minimum Abbe number decreases below the lower limit value of Conditional Expression (4), the Abbe numbers of all the positive lenses in the fifth lens group Gare decreased in conjunction with Conditional Expression (3), and the degrees of freedom of correction of chromatic aberration and other various aberrations are decreased, which makes it difficult to achieve favorable aberration correction.

In Conditional Expression (4), the effect described above can be made more reliable by desirably limiting the lower limit value thereof to 20.00.

Furthermore, in the imaging optical system of the present invention, the fourth lens group Gconsists of one or two positive lenses and one negative lens. The fourth lens group Ghas a positive refractive power. However, by disposing a negative lens and suppressing chromatic aberration in the fourth lens group G, it is easy to suppress fluctuation in chromatic aberration during focusing.

In a case where the number of lenses of the focus lens group is large and the weight is large, there is a disadvantage in the speedup of auto focus and the quietness during focus driving, and a large actuator is required, making it difficult to reduce the size and weight of the imaging lens. Therefore, it is desirable that the positive lens consists of one or two positive lenses and the negative lens consists of one negative lens.

Furthermore, in the imaging optical system of the present invention, the second lens group Gand the fourth lens group Gmove to the object side along optical axes on different trajectories during focusing from the infinite distance object to the close distance object. By increasing the degree of freedom of the movement amounts of the second lens group Gand the fourth lens group G, it is easier to suppress the fluctuation in various aberrations during focusing.

Furthermore, the imaging optical system of the present invention satisfies the conditional expression below.

The fourth lens group Ghaving a positive refractive power, which is disposed closer to the image side than the aperture diaphragm S, is responsible for most of the focusing action, and thus, it is easy to suppress focus breathing during focusing. Therefore, it is preferable that the focus sensitivity of the second lens group Gis set to a value smaller than that of the fourth lens group Gafter the focus sensitivity of the fourth lens group Gis appropriately set. Conditional Expression (5) specifies a preferable range of the focus sensitivity of the fourth lens group G.

In a case where the focus sensitivity of the fourth lens group Gis decreased below the lower limit value of Conditional Expression (5), the amount of movement required to satisfy the desired shortest focusing distance increases, and it is difficult to reduce the size and weight of the imaging lens due to an increase in the total length of the lens for securing the movement space or an increase in the size of the actuator. On the other hand, in a case where the focus sensitivity of the fourth lens group Gis increased beyond the upper limit value of Conditional Expression (5), the positive refractive power of the fourth lens group Gis increased, and the eccentricity sensitivity of the fourth lens group Gis increased, that is, the amount of deterioration in performance in a case of being eccentric is increased. The fourth lens group Gis likely to be eccentric in order to be driven during focusing, and in a case where the eccentricity sensitivity of the fourth lens group Gexceeds the upper limit and becomes large, it is difficult to suppress the individual variation in the image formation performance of the imaging lens.

In Conditional Expression (5), by desirably limiting the lower limit value to 0.65 and the upper limit value to 2.00, the above-described effect can be made more reliable.

Conditional Expression (6) specifies a preferable range for a ratio between the focus sensitivity of the second lens group Gand the focus sensitivity of the fourth lens group G.

In a case where the ratio of the focus sensitivity of the second lens group Gis increased beyond the upper limit value of Conditional Expression (6), it is difficult to sufficiently suppress focus breathing during focusing.

In Conditional Expression (6), desirably, by limiting the upper limit value to 0.40, the above-described effect can be made more reliable.

Furthermore, in the imaging optical system of the present invention, the third lens group Gincludes at least two positive lenses and at least two negative lenses. By increasing the number of positive lenses and negative lenses in the third lens group Gin which the on-axis luminous flux diameter increases in the vicinity of the aperture diaphragm S, it is easy to satisfactorily correct various aberrations, particularly, longitudinal chromatic aberration, spherical aberration, and comatic aberration even in the imaging optical system having a large aperture ratio.